In the Gospel of Matthew it says, "There is one who is your teacher...Do not allow anyone to call you teacher." The Gloss on this passage says, "Do not let this divine honor be attributed to you or any human, or else you will be usurping what belongs only to God." Therefore, only God can teach or be called teacher.

Also, if humans can teach, it is only through using signs and symbols. Now one might think that teaching can take place through the things themselves and not through signs or symbols. A common example is if someone wanted to know what "ambulare" means and you show him by walking. But even in this case you will have to use some kind of sign in order to teach him. Augustine proves this in On the Teacher by pointing out that, since one thing can have all sorts of features, we can not know what the demonstration is about. Is it about walking itself or some feature, like speed? Therefore it seems that some kind of sign is necessary. But signs are not enough. For knowledge of the signs is not knowledge of the things themselves. In fact, one must know the things themselves before understanding the signs. Signs and symbols are invented in order to communicate the knowledge of things. Therefore, no one can teach another knowledge of the things themselves.

Also, consider the case where someone uses certain signs with you. Now either you already know what those signs signify or you do not. If you do know those things that the signs are about, then the signs won't teach you about them. If you don't know them, then you couldn't know what the signs mean. For example, if you don't know what a stone is, then how could you know what the word stone means? Someone ignorant of what the signs mean can't be taught through those signs. Therefore, if all we have are signs by which to teach, then we can't teach each other.

Also, to teach is to cause knowledge in another person. But knowledge is in the intellect, whereas signs are received by the five senses. If signs are the only tools we use to teach, then our "teaching" would stop at the senses and not extend to the intellect. Therefore, one person cannot really teach another.

Let us suppose that one person's knowledge is caused by the knowledge in another. Now either that knowledge already exists in the individual who is to be taught, or it doesn't. If it doesn't, then we have the case of one person causing knowledge in another. But this is impossible. If it does already exist, either it exists in a fully actual state (which would make teaching superfluous) or in a seminal state. But no created power can make something in a seminal state to become fully actual. Only God can do this. Therefore, humans cannot teach each other.

Also, knowledge is nothing other than the representation of things in the soul. For knowledge is the conformity of the knower to the known. But one human cannot make a copy of things in the soul of another, because a human cannot change the inside of another. Only God can. Therefore, one man cannot teach another.

Also, Boethius says in On the Consolation of Philosophy that teaching only inspires the human mind to know things. But inspiring the mind is different from making it to know, just as helping a person to see with his eyes is different from making him to see. Therefore, one person does make another to know, and so does not really teach him.

On the other hand, as Aristotle says in Book IV of his Meteorology (and Book IX of his Metaphysics), everything reaches a perfect state when it can produce another like to itself. But knowledge is a perfect state of the mind. Therefore, someone who has knowledge can teach another.

My response to this question is as follows. First we should note that the same kind of disagreement is found among these three questions: how something comes into existence, how someone acquires virtue, and how someone comes to know.

I. Some people have said that things come into existence by virtue of an outside power, which they call the "giver of forms" and "intelligent cause." All other causes are merely instruments that prepare the material for the forms.

In a similar way, Avicenna says in Metaphysics that the virtue of honesty is not acquired by our attempts at being honest. Instead, our attempts only keep us away from dishonesty and help prepare us to receive the quality of honesty from the intelligent cause.

II. On the other hand, some have said that all things that seem to come into existence already exist but are initially hidden. The only thing natural forces have to do is bring these hidden qualities to light.

In a similar way, some have held that humans are born with all the virtues. All that needs to be done is to remove the impediments that keep the good qualities hidden, just as someone might sand away rust in order to reveal the brightness of iron.

Likewise, some have said that the human soul is created with the knowledge of all things. According to this opinion, teaching is nothing other than helping the soul to remember and consider the things it already knows. So they say that learning is remembering.

For the first position rules out any possibility of a chain of causes, since the first cause, such as the "giver of forms", is the only cause. But this opinion insults the universe, which is woven together by the order and connection of causes. For the primary cause, from its outstanding goodness, makes other things not only to be, but also to be causes.

The second position falls down for the same reason. For a cause that only removes an impediment is not a cause in the truest sense. So, if all causes only take away whatever is obscuring natural features, virtues, or knowledge, then there are no true causes in the world.

For natural forms and features do preexist, but not actually, as some say, but potentially. They are brought into being from their potential states by causes outside them but close to them, and not only by the supreme cause.

In a similar way, before virtuous qualities are fully actual in us they preexist in our natural inclinations, which are the seeds of virtue. Through practice they are brought to their proper completion.

The same sort of thing should be said about learning and teaching. The seeds of knowledge preexist in us. These are the elementary ideas (both simple and complex) which we understand immediately by the light of our own minds. From these common principles all other principles of knowledge grow, like a plant grows from a seed. Therefore, when from these common notions a mind moves into an actual state of knowing some more specific things (which before it knew only potentially and at a very general level), then someone can be said to have acquired knowledge.

First, something can preexist potentially in a complete and active way, when, namely, an intrinsic principle is enough to make it become fully actual. This is what often happens in the process of getting healthy, as when from the natural power within the sick person, his health can be recovered.

Second, something can preexist potentially in a passive way, such that an intrinsic principle is not enough to bring it into full existence. This is the case when fire comes into existence from air, not from any intrinsic power of the air.

When something preexists in the first way, then an outside cause helps the intrinsic cause by providing it with the things it needs to do its work. For example, when a doctor heals he is in fact ministering to nature, which is the principal cause. He adapts his practice to the needs of nature and by using medicines or other instruments provides nature with what it needs to bring about health.

VI. Therefore, just as someone can be healed in two ways -- first by the action of nature only, second by the collaboration of nature and medicine -- so also there are two ways of acquiring knowledge. First, when the mind moves by its own natural power to an understanding of things previously unknown to it. This is called discovery (inventio). Second, when the mind is helped by an outside power of reason. This is called teaching (disciplina).

Now in those things that come about by nature and art, art works in the same way and uses the same sorts of tools as nature. For just as nature uses warmth to heal someone suffering from a cold, so also does a doctor. This is why art is said to imitate nature. Similarly, in the acquisition of knowledge, the teacher leads the student to the knowledge of things the student previously did not know in the same way that someone leads himself to discover what he previously did not know.

The process of discovery begins with applying common self-evident principles to particular subject matters, and then proceeding to some particular conclusions, and then from these moving on to other conclusions. In light of this, one is said to teach another, when he makes clear through certain signs the path (discursum) of reasoning he himself took. Thus the teacher's presentations are like tools that the natural reason of the student uses to come to an understanding of things previously unknown to him.

Therefore, just as the doctor is said to cause health in the sick man with nature working, so also one is said to cause knowledge in another by the activity of the power of reasoning in that person, and this is called teaching. In this way one person is said to teach another and to be his teacher. Thus the Philosopher says that a demonstration is a syllogism causing knowledge.

Now if someone proposes to another certain ideas that are not self-evident or if he does not manifest how they follow from self-evident principles, then he does not cause knowledge in that person, but rather opinion or belief. For those ideas that follow necessarily from the first self
evident principles have to be true, and those that are contrary to them have to be false. But to all other ideas he can give his assent or not.

Finally, we should note that the light of reason, by which the first principles are known to us, is endowed by God. It is in fact a kind of likeness of the uncreated truth. Thus, since every human teaching has its validity only through the power of that divine light, then it follows that God alone is the most interior and principal teacher of all, just as nature is the principal and interior healer. But this does not rule out that teaching and healing can be spoken of in the way I have already mentioned.

My responses to the objections are as follows: To the first we should say that God's command to his disciples not to let themselves be called teacher is not an absolute prohibition. We are prohibited to call someone a teacher as if he were the supreme teacher. The point is that we should not place unlimited trust in human wisdom, but only in the divine truth which speaks in us through an impression in us of its likeness and by which we can judge all things.

To the second we should say that our knowledge of things is not brought about through the understanding of the signs, but through the understanding of things more certain, such as the principles (which are presented to us through signs). These principles are applied to other things that we were previously ignorant of, even though we did know them in some way (as I have explained above). For the understanding of the principles, not of the signs, gives us knowledge of the conclusions.

To the third we should point out that, when we are being taught certain things through signs, we must already understand those things in some way, while being ignorant of them in other ways. For example, if we are being taught about humans, we have to already know something about being human, either that a human is a thinking animal, or some kind of substance, or at least some kind of being (which everyone already must know). Likewise, if we are learning a proof, we must already have some understanding of the subject of the proof and what is trying to be proved about the subject, in addition to prior knowledge of the first principles. For all learning is based on preexisting knowledge. Thus, the conclusion of this objection doesn't follow.

To the fourth we should say that the human mind takes ideas from the signs that our senses perceive. It then uses these to produce knowledge in itself. For signs are not the immediate cause of knowledge, but the mind moving from principles to conclusions, as I said above.

To the fifth we should reply that knowledge does preexist in the student, not completely but in a seminal state. For the common notions, which are present in our mind by nature, are like seeds of all the knowledge that will follow. Now while it is true that no created power has implanted these knowledge "seeds" in us, still the action of a created power can realize the potential of those seeds.

To the eleventh we should say that the student's ideas are the primary foundation on which is built all the knowledge gained through teaching. The student's own lights are the immediate builder, while the teacher's are the middle builder. For the teacher presents signs of the knowable things, from which the student's mind takes ideas in order to consider them. Thus the teacher's words or writings end up being like the subject of study, since the student takes ideas from both. The difference is that the teacher's words are a more direct way of generating knowledge than the experience of the subject since they are signs of the ideas themselves.

Eyesight is not a comparative power (that is, one capable of becoming aware of some things by looking at others). For all of the objects of sight are visible to it as soon as it turns toward them. It is much like someone who has already acquired knowledge of something -- in order to think it, the person merely needs to turn his attention to it. Thus, we do not need others to inspire or rouse up our vision, unless perhaps when we need someone to point out an object for us to see.

The mind, on the other hand, is a comparative power. Not all things are equally understandable to it. For certain things, such as the first self-evident principles it see immediately. But those ideas that follow from the principles and are implicitly contained in them need the process of reasoning and demonstration before the mind can see them. In this case the mind that is seeking to know these does need the help of teaching, unlike a mind that has already understood these ideas but is not thinking about them at the moment.

Therefore, the teacher inspires the student to understand what he is teaching, just as any cause can realize the potential in another thing. But when someone presents something to the sense of sight, that person is more of a secondary (per accidens) cause that helps realize potential.